Addiction of MYCN amplified tumours to B-MYB underscores a reciprocal regulatory loop

MYB oncoproteins: emerging players and potential therapeutic targets in human cancer

MYB transcription factors are highly conserved from plants to vertebrates, indicating that their functions embrace fundamental mechanisms in the biology of cells and organisms. In humans, the MYB gene family is composed of three members: MYB, MYBL1 and MYBL2, encoding the transcription factors MYB, MYBL1, and MYBL2 (also known as c-MYB, A-MYB, and B-MYB), respectively. A truncated version of MYB, the prototype member of the MYB family, was originally identified as the product of the retroviral oncogene v-myb, which causes leukaemia in birds. This led to the hypothesis that aberrant activation of vertebrate MYB could also cause cancer. Despite more than three decades have elapsed since the isolation of v-myb, only recently investigators were able to detect MYB genes rearrangements and mutations, smoking gun evidence of the involvement of MYB family members in human cancer. In this review, we will highlight studies linking the activity of MYB family members to human malignancies and experimental therapeutic interventions tailored for MYB-expressing cancers.

Burkholderia Lethal Factor 1, a Novel Anti-Cancer Toxin, Demonstrates Selective Cytotoxicity in MYCN-Amplified Neuroblastoma Cells

Immunotoxins are being investigated as anti-cancer therapies and consist of a cytotoxic enzyme fused to a cancer targeting antibody. All currently used toxins function via the inhibition of protein synthesis, making them highly potent in both healthy and transformed cells. This non-specific cell killing mechanism causes dose-limiting side effects that can severely limit the potential of immunotoxin therapy. In this study, the recently characterised bacterial toxin Burkholderia lethal factor 1 (BLF1) is investigated as a possible alternative payload for targeted toxin therapy in the treatment of neuroblastoma. BLF1 inhibits translation initiation by inactivation of eukaryotic initiation translation factor 4A (eIF4A), a putative anti-cancer target that has been shown to regulate a number of oncogenic proteins at the translational level. We show that cellular delivery of BLF1 selectively induces apoptosis in neuroblastoma cells that display MYCN amplification but has little effect on non-transformed cells. Future immunotoxins based on this enzyme may therefore have higher specificity towards MYCN-amplified cancer cells than more conventional ribosome-inactivating proteins, leading to an increased therapeutic window and decreased side effects.

Cross-Cohort Analysis Identifies a TEAD4-MYCN Positive Feedback Loop as the Core Regulatory Element of High-Risk Neuroblastoma

High-risk neuroblastomas show a paucity of recurrent somatic mutations at diagnosis. As a result, the molecular basis for this aggressive phenotype remains elusive. Recent progress in regulatory network analysis helped us elucidate disease-driving mechanisms downstream of genomic alterations, including recurrent chromosomal alterations. Our analysis identified three molecular subtypes of high-risk neuroblastomas, consistent with chromosomal alterations, and identified subtype-specific master regulator proteins that were conserved across independent cohorts. A 10-protein transcriptional module-centered around a TEAD4-MYCN positive feedback loop-emerged as the regulatory driver of the high-risk subtype associated with MYCN amplification. Silencing of either gene collapsed MYCN-amplified (MYCNAmp) neuroblastoma transcriptional hallmarks and abrogated viability in vitro and in vivo Consistently, TEAD4 emerged as a robust prognostic marker of poor survival, with activity independent of the canonical Hippo pathway transcriptional coactivators YAP and TAZ. These results suggest novel therapeutic strategies for the large subset of MYCN-deregulated neuroblastomas.Significance: Despite progress in understanding of neuroblastoma genetics, little progress has been made toward personalized treatment. Here, we present a framework to determine the downstream effectors of the genetic alterations sustaining neuroblastoma subtypes, which can be easily extended to other tumor types. We show the critical effect of disrupting a 10-protein module centered around a YAP/TAZ-independent TEAD4-MYCN positive feedback loop in MYCNAmp neuroblastomas, nominating TEAD4 as a novel candidate for therapeutic intervention. Cancer Discov; 8(5); 582-99. ©2018 AACR.This article is highlighted in the In This Issue feature, p. 517.

MYBL2 protects against H9c2 injury induced by hypoxia via AKT and NF‑κB pathways

Cardiovascular diseases have become one of the major public health problems in many countries. The downregulation of MYBL2 was found in H9c2 and native cardiomyocytes cells after hypoxia treatment. The present study aimed to investigate the effects of MYB proto‑oncogene like 2 (MYBL2) on H9c2 injury induced by hypoxia. Reverse transcription‑quantitative polymerase chain reaction and western blot were performed on H9c2 cells to determine the mRNA and protein levels of MYBL2, respectively. Small interfering RNA (siRNA) was employed to downregulate MYBL2 expression in H9c2 cells to investigate changes in cell proliferation and apoptosis. Cell proliferation was assessed by a Cell Counting kit‑8 assay and the percentage of apoptotic cells was determined using an Annexin V‑fluorescein isothiocyanate/propidium iodide apoptosis detection kit. The nuclear factor‑κB (NF‑κB) and AKT signaling pathways in H9c2 cells were investigated by western blot analysis. The results demonstrated that the overexpression of MYBL2 promoted cell proliferation and suppressed apoptosis. Furthermore, overexpression of MYBL2 suppressed the expression of phosphorylated (p)‑AKT, p‑NF‑κB inhibitor α, p‑p65 and B‑cell CLL/lymphoma 3 (Bcl‑3). The results indicated that MYBL2 may improve cell viability and inhibit H9c2 apoptosis via the inhibition of AKT and NF‑κB pathways. Therefore, MYBL2 may be a potential therapeutic target for the treatment of myocardial infarction.

Unveiling MYCN regulatory networks in neuroblastoma via integrative analysis of heterogeneous genomics data

MYCN, an oncogenic transcription factor of the Myc family, is a major driver of neuroblastoma tumorigenesis. Due to the difficulty in drugging MYCN directly, revealing the molecules in MYCN regulatory networks will help to identify effective therapeutic targets for neuroblastoma therapy. Here we perform ChIP-sequencing and small RNA-sequencing of neuroblastoma cells to determine the MYCN-binding sites and MYCN-associated microRNAs, and integrate various types of genomic data to construct MYCN regulatory networks. The overall analysis indicated that MYCN-regulated genes were involved in a wide range of biological processes and could be used as signatures to identify poor-prognosis MYCN-non-amplified patients. Analysis of the MYCN binding sites showed that MYCN principally served as an activator. Using a computational approach, we identified 32 MYCN co-regulators, and some of these findings are supported by previous studies. Moreover, we investigated the interplay between MYCN transcriptional and microRNA post-transcriptional regulations and identified several microRNAs, such as miR-124-3p and miR-93-5p, which may significantly contribute to neuroblastoma pathogenesis. We also found MYCN and its regulated microRNAs acted together to repress the tumor suppressor genes. This work provides a comprehensive view of MYCN regulations for exploring therapeutic targets in neuroblastoma, as well as insights into the mechanism of neuroblastoma tumorigenesis.

Cinacalcet inhibits neuroblastoma tumor growth and upregulates cancer-testis antigens

The calcium–sensing receptor is a G protein-coupled receptor that exerts cell-type specific functions in numerous tissues and some cancers. We have previously reported that this receptor exhibits tumor suppressor properties in neuroblastoma. We have now assessed cinacalcet, an allosteric activator of the CaSR approved for clinical use, as targeted therapy for this developmental tumor using neuroblastoma cell lines and patient-derived xenografts (PDX) with different MYCN and TP53 status. In vitro, acute exposure to cinacalcet induced endoplasmic reticulum stress coupled to apoptosis via ATF4-CHOP-TRB3 in CaSR-positive, MYCN-amplified cells. Both phenotypes were partially abrogated by phospholipase C inhibitor U73122. Prolonged in vitro treatment also promoted dose- and time-dependent apoptosis in CaSR-positive, MYCN-amplified cells and, irrespective of MYCN status, differentiation in surviving cells. Cinacalcet significantly inhibited tumor growth in MYCN-amplified xenografts and reduced that of MYCN-non amplified PDX. Morphology assessment showed fibrosis in MYCN-amplified xenografts exposed to the drug. Microarrays analyses revealed up-regulation of cancer-testis antigens (CTAs) in cinacalcet-treated MYCN-amplified tumors. These were predominantly CTAs encoded by genes mapping on chromosome X, which are the most immunogenic. Other modulated genes upon prolonged exposure to cinacalcet were involved in differentiation, cell cycle exit, microenvironment remodeling and calcium signaling pathways. CTAs were up-regulated in PDX and in vitro models as well. Moreover, progressive increase of CaSR expression upon cinacalcet treatment was seen both in vitro and in vivo. In summary, cinacalcet reduces neuroblastoma tumor growth and up-regulates CTAs. This effect represents a therapeutic opportunity and provides surrogate circulating markers of neuroblastoma response to this treatment.

PRC2-Mediated Transcriptomic Alterations at the Embryonic Stage Govern Tumorigenesis and Clinical Outcome in MYCN-Driven Neuroblastoma

Pediatric cancers such as neuroblastoma are thought to involve a dysregulation of embryonic development. However, it has been difficult to identify the critical events that trigger tumorigenesis and differentiate them from normal development. In this study, we report the establishment of a spheroid culture method that enriches early-stage tumor cells from TH-MYCN mice, a preclinical model of neuroblastoma. Using this method, we found that tumorigenic cells were evident as early as day E13.5 during embryo development, when the MYC and PRC2 transcriptomes were significantly altered. Ezh2, an essential component of PRC2, was expressed in embryonic and postnatal tumor lesions and physically associated with N-MYC and we observed that H3K27me3 was increased at PRC2 target genes. PRC2 inhibition suppressed in vitro sphere formation, derepressed its target genes, and suppressed in situ tumor growth. In clinical specimens, expression of MYC and PRC2 target genes correlated strongly and predicted survival outcomes. Together, our findings highlighted PRC2-mediated transcriptional control during embryogenesis as a critical step in the development and clinical outcome of neuroblastoma. Cancer Res; 77(19); 5259-71. ©2017 AACR.

MYBL2 (B-Myb): a central regulator of cell proliferation, cell survival and differentiation involved in tumorigenesis

Limitless cell proliferation, evasion from apoptosis, dedifferentiation, metastatic spread and therapy resistance: all these properties of a cancer cell contribute to its malignant phenotype and affect patient outcome. MYBL2 (alias B-Myb) is a transcription factor of the MYB transcription factor family and a physiological regulator of cell cycle progression, cell survival and cell differentiation. When deregulated in cancer cells, MYBL2 mediates the deregulation of these properties. In fact, MYBL2 is overexpressed and associated with poor patient outcome in numerous cancer entities. MYBL2 and players of its downstream transcriptional network can be used as prognostic and/or predictive biomarkers as well as potential therapeutic targets to offer less toxic and more specific anti-cancer therapies in future. In this review, we summarize current knowledge on the physiological roles of MYBL2 and highlight the impact of its deregulation on cancer initiation and progression.

MYCN induces neuroblastoma in primary neural crest cells

Neuroblastoma (NBL) is an embryonal cancer of the sympathetic nervous system (SNS), which causes 15% of pediatric cancer deaths. High-risk NBL is characterized by N-Myc amplification and segmental chromosomal gains and losses. Owing to limited disease models, the etiology of NBL is largely unknown, including both the cell of origin and the majority of oncogenic drivers. We have established a novel system for studying NBL based on the transformation of neural crest cells (NCCs), the progenitor cells of the SNS, isolated from mouse embryonic day 9.5 trunk neural tube explants. Based on pathology and gene expression analysis, we report the first successful transformation of wild-type NCCs into NBL by enforced expression of N-Myc, to generate phenotypically and molecularly accurate tumors that closely model human MYCN-amplified NBL. Using comparative genomic hybridization, we found that NCC-derived NBL tumors acquired copy number gains and losses that are syntenic to those observed in human MYCN-amplified NBL including 17q gain, 2p gain and loss of 1p36. When p53-compromised NCCs were transformed with N-Myc, we generated primitive neuroectodermal tumors with divergent differentiation including osteosarcoma. These subcutaneous tumors were metastatic to regional lymph nodes, liver and lung. Our novel experimental approach accurately models human NBL and establishes a new system with potential to study early stages of NBL oncogenesis, to functionally assess NBL oncogenic drivers and to characterize NBL metastasis.

Cutaneous cylindroma: it's all about MYB

Cutaneous cylindroma is a rare benign tumour that occasionally turns into malignant cylindrocarcinoma. The cancer can be sporadic or emerge in the context of Brooke-Spiegler syndrome (BSS), an inheritable condition characterized by mutation of the gene CYLD, encoding a tumour suppressor protein that controls the activity of the transcription factor NF-kB. Sporadic cylindromas present histological features shared with adenoid cystic carcinoma (ACC), a head and neck cancer originating from salivary or other exocrine glands. Like ACCs, sporadic cylindromas express, although at lower frequency, the aberrant fusion transcript MYB-NFIB. In a paper recently published in the Journal of Pathology, the research teams led by Neil Rajan and Goran Stenman demonstrate that CYLD-defective cyclindromas in BSS patients are negative for the MYB-NFIB fusion. Only the wild-type MYB oncoprotein is activated in the majority of these tumours. RNA interference studies in cells derived from BSS patients indicate that ablating MYB expression results in a striking reduction of cylindroma cell proliferation, suggesting that MYB plays a pivotal role in the biology of this cancer. The take-home message of the study is that activation of MYB, in its wild-type form or fusion derivatives, is a common feature of spontaneous and hereditary cylindromas, constituting a potentially actionable therapeutic target. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Regulation of Differentiation by Calcium-Sensing Receptor in Normal and Tumoral Developing Nervous System

During normal development of the nervous system (NS), neural progenitor cells (NPCs) produce specialized populations of neurons and glial cells upon cell fate restriction and terminal differentiation. These sequential processes require the dynamic regulation of thousands of genes. The calcium-sensing receptor (CaSR) is temporally and spatially regulated in both neurons and glial cells during development of the NS. In particular, CaSR expression and function have been shown to play a significant role during differentiation of NPCs toward the oligodendrocyte lineage and also in maturation of cerebellar granule cell precursors (GCPs). Moreover, CaSR regulates axonal and dendritic growth in both central and peripheral nervous systems (PNSs), a process necessary for proper construction of mature neuronal networks. On the other hand, several lines of evidence support a role for CaSR in promotion of cell differentiation and inhibition of proliferation in neuroblastoma, a tumor arising from precursor cells of developing PNS. Thus, among the variety of NS functions in which the CaSR participates, this mini-review focuses on its role in differentiation of normal and tumoral cells. Current knowledge of the mechanisms responsible for CaSR regulation and function in these contexts is also discussed, together with the therapeutic opportunities provided by CaSR allosteric modulators.

Chorein addiction in VPS13A overexpressing rhabdomyosarcoma cells

Chorein encoded by VPS13A (vacuolar protein sorting-associated protein 13A) is defective in chorea-acanthocytosis. Chorein fosters neuronal cell survival, cortical actin polymerization and cell stiffness. In view of its anti-apoptotic effect in neurons, we explored whether chorein is expressed in cancer cells and influences cancer cell survival. RT-PCR was employed to determine transcript levels, specific siRNA to silence chorein, FACS analysis to follow apoptosis and Western blotting to quantify protein abundance. Chorein transcripts were detected in various cancer cell types. The mRNA coding for chorein and chorein protein were most abundant in drug resistant, poorly differentiated human rhabdomyosarcoma cells. Chorein silencing significantly reduced the ratio of phosphorylated (and thus activated) to total phosphoinositide 3 kinase (PI-3K), pointing to inactivation of this crucial pro-survival signaling molecule. Moreover, chorein silencing diminished transcript levels and protein expression of anti-apoptotic BCL-2 and enhanced transcript levels of pro-apoptotic Bax. Silencing of chorein in rhabdomyosarcoma cells was followed by mitochondrial depolarization, caspase 3 activation and stimulation of early and late apoptosis. In conclusion, chorein is expressed in various cancer cells. In cells with high chorein expression levels chorein silencing promotes apoptotic cell death, an effect paralleled by down-regulation of PI-3K activity and BCL-2/Bax expression ratio.

The stress phenotype makes cancer cells addicted to CDT2, a substrate receptor of the CRL4 ubiquitin ligase

CDT2/L2DTL/RAMP is one of the substrate receptors of the Cullin Ring Ubiquitin Ligase 4 that targets for ubiquitin mediated degradation a number of substrates, such as CDT1, p21 and CHK1, involved in the regulation of cell cycle and survival. Here we show that CDT2 depletion was alone able to induce the apoptotic death in 12/12 human cancer cell lines from different tissues, regardless of the mutation profile and CDT2 expression level. Cell death was associated to rereplication and to loss of CDT1 degradation. Conversely, CDT2 depletion did not affect non-transformed human cells, such as immortalized kidney, lung and breast cell lines, and primary cultures of endothelial cells and osteoblasts. The ectopic over-expression of an activated oncogene, such as the mutation-activated RAS or the amplified MET in non-transformed immortalized breast cell lines and primary human osteoblasts, respectively, made cells transformed in vitro, tumorigenic in vivo, and susceptible to CDT2 loss. The widespread effect of CDT2 depletion in different cancer cells suggests that CDT2 is not in a synthetic lethal interaction to a single specific pathway. CDT2 likely is a non-oncogene to which transformed cells become addicted because of their enhanced cellular stress, such as replicative stress and DNA damage.

High Id1 expression, a generally negative prognostic factor, paradoxically predicts a favorable prognosis for adjuvant paclitaxel plus cisplatin therapy in surgically treated lung cancer patients

Adjuvant chemotherapy is commonly given to surgically treated non-small-cell lung cancer (NSCLC) patients. However, the prerequisite for chemotherapy needs to be scrutinized in order to maximize the benefits to patients. In this study, we observed that NSCLC cells with high Id1 protein expression were vulnerable to the treatment of paclitaxel and cisplatin. In addition, paclitaxel and cisplatin caused Id1 protein degradation through ubiquitination. In the nude mice xenograft model, the tumor growth was reduced to a large degree in the Id1-overexpressing group upon treatment with paclitaxel and cisplatin. Furthermore, immunohistochemical staining for Id1 followed by Kaplan-Meier survival analysis showed that surgically treated NSCLC patients with high Id1 expression in primary tumor tissues had better disease-free and overall survivals after adjuvant paclitaxel and cisplatin chemotherapy. In summary, our current data suggest that Id1, a generally negative prognostic factor, predicts a favorable prognosis in the case of surgically treated NSCLC patients receiving the definitive adjuvant chemotherapy. The distinct role of Id1 reported in this study may arise from the phenomenon of Id1 dependence of NSCLC cells for survival, which renders the cancer cells additionally susceptive to the adjuvant chemotherapy with paclitaxel and cisplatin.

Identification and pharmacological inactivation of the MYCN gene network as a therapeutic strategy for neuroblastic tumor cells

The MYC family of transcription factors consists of three well characterized members, c-MYC, L-MYC, and MYCN, deregulated in the majority of human cancers. In neuronal tumors such as neuroblastoma, MYCN is frequently activated by gene amplification, and reducing its expression by RNA interference has been shown to promote growth arrest and apoptosis of tumor cells. From a clinical perspective, RNA interference is not yet a viable option, and small molecule inhibitors of transcription factors are difficult to develop. We therefore planned to identify, at the global level, the genes interacting functionally with MYCN required to promote fitness of tumor cells facing oncogenic stress. To find genes whose inactivation is synthetically lethal to MYCN, we implemented a genome-wide approach in which we carried out a drop-out shRNA screen using a whole genome library that was delivered into isogenic neuroblastoma cell lines expressing or not expressing MYCN. After the screen, we selected for in-depth analysis four shRNAs targeting AHCY, BLM, PKMYT1, and CKS1B. These genes were chosen because they are directly regulated by MYC proteins, associated with poor prognosis of neuroblastoma patients, and inhibited by small molecule compounds. Mechanistically, we found that BLM and PKMYT1 are required to limit oncogenic stress and promote stabilization of the MYCN protein. Cocktails of small molecule inhibitors of CKS1B, AHCY, BLM, and PKMYT1 profoundly affected the growth of all neuroblastoma cell lines but selectively caused death of MYCN-amplified cells. Our findings suggest that drugging the MYCN network is a promising avenue for the treatment of high risk, neuroblastic cancers.

Cidofovir selectivity is based on the different response of normal and cancer cells to DNA damage

Background

Cidofovir (CDV) proved efficacious in treatment of human papillomaviruses (HPVs) hyperplasias. Antiproliferative effects of CDV have been associated with apoptosis induction, S-phase accumulation, and increased levels of tumor suppressor proteins. However, the molecular mechanisms for the selectivity and antitumor activity of CDV against HPV-transformed cells remain unexplained.

Methods

We evaluated CDV drug metabolism and incorporation into cellular DNA, in addition to whole genome gene expression profiling by means of microarrays in two HPV⁺ cervical carcinoma cells, HPV^- immortalized keratinocytes, and normal keratinocytes.

Results

Determination of the metabolism and drug incorporation of CDV into genomic DNA demonstrated a higher rate of drug incorporation in HPV⁺ tumor cells and immortalized keratinocytes compared to normal keratinocytes. Gene expression profiling clearly showed distinct and specific drug effects in the cell types investigated. Although an effect on inflammatory response was seen in all cell types, different pathways were identified in normal keratinocytes compared to immortalized keratinocytes and HPV⁺ tumor cells. Notably, Rho GTPase pathways, LXR/RXR pathways, and acute phase response signaling were exclusively activated in immortalized cells. CDV exposed normal keratinocytes displayed activated cell cycle regulation upon DNA damage signaling to allow DNA repair via homologous recombination, resulting in genomic stability and survival. Although CDV induced cell cycle arrest in HPV^- immortalized cells, DNA repair was not activated in these cells. In contrast, HPV⁺ cells lacked cell cycle regulation, leading to genomic instability and eventually apoptosis.

Conclusions

Taken together, our data provide novel insights into the mechanism of action of CDV and its selectivity for HPV-transformed cells. The proposed mechanism suggests that this selectivity is based on the inability of HPV⁺ cells to respond to DNA damage, rather than on a direct anti-HPV effect. Since cell cycle control is deregulated by the viral oncoproteins E6 and E7 in HPV⁺ cells, these cells are more susceptible to DNA damage than normal keratinocytes. Our findings underline the therapeutic potential of CDV for HPV-associated malignancies as well as other neoplasias.

A chemical screen identifies the chemotherapeutic drug topotecan as a specific inhibitor of the B-MYB/MYCN axis in neuroblastoma

The transcription factor MycN is the prototypical neuroblastoma oncogene and a potential therapeutic target. However, its strong expression caused by gene amplification in about 30% of neuroblastoma patients is a considerable obstacle to the development of therapeutic approaches aiming at eliminating its tumourigenic activity. We have previously reported that B-Myb is essentially required for transcription of the MYCN amplicon and have also shown that B-MYB and MYCN are engaged in a feed forward loop promoting the survival/proliferation of neuroblastoma cells. We postulated that pharmacological strategies breaking the B-MYB/MYCN axis should result in clinically desirable effects. Thus, we implemented a high throughput chemical screen, using a curated library of ~1500 compounds from the National Cancer Institute, whose endpoint was the identification of small molecules that inhibited B-Myb. At the end of the screening, we found that the compounds pinafide, ellipticine and camptothecin inhibited B-Myb transcriptional activity in luciferase assays. One of the compounds, the topoisomerase-1 inhibitor camptothecin, is of considerable clinical interest since its derivatives topotecan and irinotecan are currently used as first and second line treatment agents for various types of cancer, including neuroblastoma. We found that neuroblastoma cells with amplification of MYCN are more sensitive than MYCN negative cells to camptothecin and topotecan killing. Campothecin and topotecan caused selective down-regulation of B-Myb and MycN expression in neuroblastoma cells. Notably, forced overexpression of B-Myb could antagonize the killing effect of topotecan and camptothecin, demonstrating that the transcription factor is a key target of the drugs. These results suggest that camptothecin and its analogues should be more effective in patients whose tumours feature amplification of MYCN and/or overexpression of B-MYB.

NCI's provocative questions on cancer: some answers to ignite discussion

National Cancer Institute has announced 24 provocative questions on cancer. Here I try to answer some of them by linking the dots of existing knowledge.

MiRNA-335 suppresses neuroblastoma cell invasiveness by direct targeting of multiple genes from the non-canonical TGF-β signalling pathway

Transforming growth factor-β (TGF-β) signaling regulates many diverse cellular activities through both canonical (SMAD-dependent) and non-canonical branches, which includes the mitogen-activated protein kinase (MAPK), Rho-like guanosine triphosphatase and phosphatidylinositol-3-kinase/AKT pathways. Here, we demonstrate that miR-335 directly targets and downregulates genes in the TGF-β non-canonical pathways, including the Rho-associated coiled-coil containing protein (ROCK1) and MAPK1, resulting in reduced phosphorylation of downstream pathway members. Specifically, inhibition of ROCK1 and MAPK1 reduces phosphorylation levels of the motor protein myosin light chain (MLC) leading to a significant inhibition of the invasive and migratory potential of neuroblastoma cells. Additionally, miR-335 targets the leucine-rich alpha-2-glycoprotein 1 (LRG1) messenger RNA, which similarly results in a significant reduction in the phosphorylation status of MLC and a decrease in neuroblastoma cell migration and invasion. Thus, we link LRG1 to the migratory machinery of the cell, altering its activity presumably by exerting its effect within the non-canonical TGF-β pathway. Moreover, we demonstrate that the MYCN transcription factor, whose coding sequence is highly amplified in a particularly clinically aggressive neuroblastoma tumor subtype, directly binds to a region immediately upstream of the miR-335 transcriptional start site, resulting in transcriptional repression. We conclude that MYCN contributes to neuroblastoma cell migration and invasion, by directly downregulating miR-335, resulting in the upregulation of the TGF-β signaling pathway members ROCK1, MAPK1 and putative member LRG1, which positively promote this process. Our results provide novel insight into the direct regulation of TGF-β non-canonical signaling by miR-335, which in turn is downregulated by MYCN.

Anaplastic Lymphoma Kinase (ALK) regulates initiation of transcription of MYCN in neuroblastoma cells

Neuroblastoma is a neural crest-derived embryonal tumour of the postganglionic sympathetic nervous system and a disease with several different chromosomal gains and losses, which include MYCN-amplified neuroblastoma on chromosome 2, deletions of parts of the chromosomes 1p and 11q, gain of parts of 17q and triploidy. Recently, activating mutations of the ALK (Anaplastic Lymphoma Kinase) RTK (Receptor Tyrosine Kinase) gene have been described in neuroblastoma. A meta-analysis of neuroblastoma cases revealed that ALK mutations (49 of 709 cases) in relation to genomic subtype were most frequently observed in MYCN amplified tumours (8.9%), correlating with a poor clinical outcome. MYCN proteins target proliferation and apoptotic pathways, and have an important role in the progression of neuroblastoma. Here, we show that both wild-type and gain-of-function mutants in ALK are able to stimulate transcription at the MYCN promoter and initiate mRNA transcription of the MYCN gene in both neuronal and neuroblastoma cell lines. Further, this stimulation of MYCN gene transcription and de novo MYCN protein expression is abrogated by specific ALK inhibitors, such as crizotinib (PF-2341066), NVP-TAE684, and by small interfering RNA to ALK resulting in a decrease in proliferation rate. Finally, co-transfection of ALK gain-of-function mutations together with MYCN leads to an increase in transformation potential. Taken together, our results indicate that ALK signalling regulates initiation of transcription of the MYCN gene providing a possible explanation for the poor clinical outcome observed when MYCN is amplified together with activated ALK.

B-Myb, cancer, senescence, and microRNAs

The transcription factor B-Myb plays a critical role in regulating gene expression and is implicated in controlling carcinogenesis and cellular senescence. Transcription of the B-Myb gene is regulated by retinoblastoma proteins acting directly on the B-Myb promoter. Recently, we found that microRNAs also control the abundance of B-Myb mRNA during senescence, adding another level of complexity to B-Myb regulation. This review focuses on the importance of B-Myb in cancer and senescence, with an emphasis on the regulation of B-Myb expression and activity.